1. Field of the Invention
The present invention relates to the remote surveillance of equipment located along a digital transmission link.
2. Description of the Prior Art
To meet the service life and reliability requirements for very long range transmission links it has seemed necessary to provide, in the context of fiber optic systems, some redundancy at the level of the laser diodes equipping the repeaters. Such redundancy achieved by equipping each repeater from the outset with a number of replacement laser diodes placed in service one by one by a switching circuit implies the existence of a system for remote controlling the switching circuits of the repeaters from the line terminal equipments and a system for transmitting signalling from each repeater to indicate to the terminal equipments the status of the laser diodes in service.
There is known from U.S. Pat. No. 4,281,416 a fiber optic digital link equipped with a remote control system for replacing in each repeater the laser diode in service by a spare laser diode and a signalling system specific to each repeater enabling an alarm to be transmitted as soon as the bias current of the laser diode in service passes a threshold. The remote control signal comprises a digital message of the same nature as the digital signal transmitted by the link and which is inserted into the traffic by the remote surveillance terminal. This digital message repeated a number of times in succession to eliminate misoperations is formed of a part identifying the repeater concerned and a part identifying in the repeater concerned the laser diode to be placed in service. The signalling signal is a pattern formed by a pseudo-random binary sequence the initial value of which identifies the source repeater.
This signalling system has the disadvantage of needing in each repeater a source of pseudo-random sequences operating at the data rate of the digital signal, meaning--in the context of optical fiber digital transmission--a very high data rate that can be achieved only using ECL technology of high power consumption. This significantly increases the power consumption of a repeater and above all its heat dissipation.
It also has the disadvantage of interrupting the link for long enough to cause loss of synchronization of the digital bit stream hierarchy so that it is only suitable for transmitting major alarms.
An object of the present invention is to alleviate these disadvantages and to achieve signalling by means of a message temporarily substituted for the data of the digital bit stream carried by the link which requires a minimum of very high speed circuits at the level of the circuits generating the message and which does not disturb the digital bit stream or the processing thereof outside the times for which the message is transmitted.